This invention relates to voltage controlled oscillator (“VCO”) circuits, and more particularly to voltage controlled oscillator circuits that can be programmed to operate in any of several frequency ranges over a very wide band of such frequency ranges.
Phase locked loop (“PLL”) circuitry is frequently used for such purposes as helping to retrieve data from a signal that may vary in frequency and/or phase. The PLL may be in relatively general-purpose circuitry such as a programmable logic device (“PLD”) that may need to support signaling at any frequency in a wide range of frequencies. (For examples of PLDs employing PLLS, see Aung et al. U.S. patent application Ser. No. 09/805,843, filed Mar. 13, 2001.) The range of frequencies over which a PLL may be required to operate is steadily increasing, particularly at the upper end (i.e., higher frequencies). At the same time, power supply voltages for integrated circuits are decreasing, making it more difficult to extend the operating range of PLL circuits.